Method for producing crystal

ABSTRACT

A method for producing a crystal, according to the present invention, where the lower surface of a seed crystal which is rotatably arranged and made of silicon carbide is brought into contact with a solution of silicon solvent containing carbon in a crucible which is rotatably arranged and the seed crystal is pulled up and a crystal of silicon carbide is grown from the solution on the lower surface of the seed crystal, comprising the steps of bringing the lower surface of the seed crystal into contact with the solution in a contact step, rotating the seed crystal in a seed crystal rotation step, rotating the crucible in a crucible rotation step, and stopping rotation of the crucible, while the seed crystal is rotated in the state in which the lower surface of the seed crystal is in contact with the solution, in a deceleration step.

TECHNICAL FIELD

The present invention relates to a method for producing a crystal toproduce a crystal of silicon carbide.

BACKGROUND ART

At present, silicon carbide (SiC) which is a compound of carbon andsilicon has attracted attention as a crystal. Silicon carbide hasattracted attention because of various advantages that, for example, theband gap is large as compared with the band gap of silicon and theelectric field strength at dielectric breakdown is large (good withstandvoltage characteristics), the thermal conductivity is high, the heatresistance is high, the chemical resistance is good, and the radiationresistance is good. A crystal of this silicon carbide is going to beapplied to fields of, for example, heavy electric equipment includingnuclear energy, transportation including automobile and aviation, homeelectrical appliance, and space. For example, Japanese Unexamined PatentApplication Publication No. 2000-264790 discloses that a single crystalof silicon carbide is produced by a solution growth method.

SUMMARY OF INVENTION

In the research and development in which a crystal made of siliconcarbide is produced by a solution growth method, it is difficult togenerate an upward flow toward a seed crystal in a solution arranged ina crucible to grow high-quality silicon carbide. The present inventionhas been devised in consideration of such circumstances and it is anobject to provide a method for producing a crystal, where high-qualitysilicon carbide can be grown.

A method for producing a crystal, according to an embodiment of thepresent invention, where the lower surface of a seed crystal which isrotatably arranged and made of silicon carbide is brought into contactwith a solution of silicon solvent containing carbon in a crucible whichis rotatably arranged and the above-described seed crystal is pulled upand a crystal of silicon carbide is grown from the above-describedsolution on the above-described lower surface of the above-describedseed crystal, includes the steps of bringing the above-described lowersurface of the above-described seed crystal into contact with theabove-described solution in a contact step, rotating the above-describedseed crystal in a seed crystal rotation step, rotating theabove-described crucible in a crucible rotation step, and deceleratingrotation of the above-described crucible and, thereafter, deceleratingrotation of the above-described seed crystal, while the above-describedlower surface of the above-described seed crystal is in contact with theabove-described solution, in a deceleration step.

According to the method for producing a crystal of the presentinvention, rotation of the seed crystal is decelerated after rotation ofthe crucible is decelerated, so that an upward flow toward the seedcrystal can be generated easily in the solution. As a result, carbon inthe solution is carried to the vicinity of the lower surface of the seedcrystal easily and, thereby, a crystal of high-quality silicon carbidecan be grown.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing an example of a crystal productionapparatus used in a method for producing a crystal, according to thepresent invention.

FIG. 2 is a magnified sectional view of a portion including a seedcrystal, an adhesive, and a holding member in the crystal productionapparatus shown in FIG. 1.

FIG. 3 is a diagram showing a rotation profile of a crucible and a seedcrystal in a method for producing a crystal, according to an embodimentof the present invention.

FIG. 4 is a schematic diagram showing a flow in a solution in a methodfor producing a crystal, according to an embodiment of the presentinvention, and is a magnified sectional view of the vicinity of acrucible and a seed crystal in the crystal production apparatus shown inFIG. 1.

FIG. 5 is a schematic diagram showing a flow in a solution in a methodfor producing a crystal, according to an embodiment of the presentinvention, (a) is a magnified sectional view of the vicinity of thecrucible and the seed crystal in the crystal production apparatus shownin FIG. 1, and (b) is a plan view of the crucible and the solution whenviewed from above.

FIG. 6 is a schematic diagram showing a flow in a solution in a methodfor producing a crystal, according to an embodiment of the presentinvention, and is a magnified sectional view of the vicinity of thecrucible and the seed crystal in the crystal production apparatus shownin FIG. 1.

FIG. 7 is a diagram showing a rotation profile of a crucible and a seedcrystal in a modified example of the method for producing a crystal,according to the present invention.

FIG. 8 is a schematic diagram showing a flow in a solution in a modifiedexample of the method for producing a crystal, according to the presentinvention.

FIG. 9 is a schematic diagram showing a change in a crystal grown by themethod for producing a crystal, shown in FIG. 8.

FIG. 10 is a diagram showing a rotation profile of a crucible and a seedcrystal in a modified example of the method for producing a crystal,according to the present invention.

FIG. 11 is a diagram showing a rotation profile of a crucible and a seedcrystal in a modified example of the method for producing a crystal,according to the present invention.

FIG. 12 is a schematic diagram showing a flow in a solution in amodified example of the method for producing a crystal, according to thepresent invention, and is a magnified sectional view of the vicinity ofthe crucible, the seed crystal, and a flow guide member in the crystalproduction apparatus shown in FIG. 1.

FIG. 13 is a plan view of a crystal production apparatus shown in FIG.12, when viewed from above (D3 direction).

FIG. 14 is a further magnified diagram of part of the crystal productionapparatus shown in FIG. 12 and is a magnified sectional view showingpart of the solution and the flow guide member.

FIG. 15 is a diagram related to a modified example of a method forproducing a crystal, according to the present invention, (a) is amagnified sectional view showing the solution and the vicinity of a flowguide member, and (b) is a plan view when viewed from above (D3direction).

FIG. 16 shows diagrams of modified examples of the method for producinga crystal, shown in FIG. 12, and each diagram shows a sectional view ofa flow guide member cut in the thickness direction.

FIG. 17 is a diagram showing a profile of the elapsed time and thesolution temperature in a modified example of the method for producing acrystal, according to the present invention.

FIG. 18 is a diagram showing a profile of the elapsed time and thesolution temperature in a modified example of the method for producing acrystal, according to the present invention.

DESCRIPTION OF EMBODIMENTS

<Crystal Production Apparatus>

Examples of a crystal production apparatus used in a method forproducing a crystal, according to the present invention, will bedescribed with reference to drawings. A crystal production apparatus 1is mainly formed from a holding member 2, an adhesive 3, a seed crystal4, and a solution 5. The outline of the crystal production apparatus 1will be described below with reference to FIG. 1.

A crucible 6 is disposed in a crucible container 7. The cruciblecontainer 7 has a function of holding the crucible 6. A heat insulatingmaterial 8 is disposed between the crucible container 7 and the crucible6. This heat insulating material 8 surrounds the crucible 6. The heatinsulating material 8 contributes to suppression of heat dissipationfrom the crucible 6 and stable keeping of the temperature of thecrucible 6.

The crucible 6 has a function of a vessel to melt, in the inside, a rawmaterial for a silicon carbide single crystal to be grown. In thepresent example, the raw material (carbon and silicon) of the singlecrystal is melted and is stored as the solution 5 in the crucible 6. Inthe present example, a solution growth method is adopted, and a crystalis grown by establishing a thermal equilibrium state in this crucible 6.

As for the crucible 6, start of rotation, acceleration of rotation,deceleration of rotation, and stop of rotation are possible. As forrotation of the crucible 6, only the crucible 6 may be rotated, thecrucible 6 and the heat insulating material 8 may be rotated, or thecrucible 6, the heat insulating material 8, and the crucible container 7may be rotated. The crucible 6 is rotated in the D1 direction(clockwise) or the D2 direction (counterclockwise), when viewed from theD3 direction. The crucible 6 may be rotated in such a way that, forexample, the barycenter of the crucible 6 serves as the center ofrotation. The crucible 6 may be rotated in such a way that the number ofrevolutions becomes, for example, 500 rpm or less.

Heat is applied to the crucible 6 by a heating mechanism 10. Anelectromagnetic heating system in which the crucible 6 is heated by anelectromagnetic wave is adopted for the heating mechanism 10 of thepresent example. The heating mechanism 10 is formed from a coil 11 andan alternating current power supply 12. The crucible 6 is made from, forexample, carbon (graphite).

The solution 5 is arranged in the crucible 6. In the solution 5, thesolvent is silicon and the solute is carbon. The solubility of anelement serving as the solute increases as the temperature of an elementserving as the solvent increases. On the other hand, if the solution 5in which the solute is dissolved into the solvent at a high temperatureis cooled, the solute more dissolved than the solubility isprecipitated. In the solution growth method adopted in the presentexample, a crystal is grown on the lower surface 4B of the seed crystal4 taking advantage of precipitation on the basis of the thermalequilibrium. The temperature of the solution 5 can be set at, forexample, 1,300° C. or higher and 2,500° C. or lower.

The solution 5 is prepared by putting the raw material for silicon (forexample, the raw material for silicon is granules) and a raw materialfor carbon (for example, raw material for carbon is granules) into thecrucible 6 and heating the raw material for silicon to melt the rawmaterial for silicon. As for the heating temperature of the crucible 6,a temperature at which the raw material for silicon is melted may beused and, for example, 1,450° C. or higher and 1,800° C. or lower can beemployed. Alternatively, the solution 5 may be prepared by melting theraw material for silicon in the crucible 6 and putting the raw materialfor carbon therein. Meanwhile, in the case where the crucible 6 is madefrom carbon, the solution 5 containing carbon can also be prepared byputting the raw material for silicon into the inside and performingmelting to dissolve the inside wall surface of the crucible 6.

In the present example, the crucible 6 is heated as described below.Initially, a current is passed through the coil 11 by using thealternating current power supply 12 to generate an electromagnetic fieldin a space including the heat insulating material 8. Then, an inducedcurrent passes through the crucible 6 on the basis of thiselectromagnetic field. The induced current passing the crucible 6 isconverted to thermal energy because of various losses, e.g., Joule heatgeneration based on electric resistance and heat generation based onhysteresis loss. That is, the crucible 6 is heated because of a heatloss of the induced current. Also, an induced current may be passedthrough the solution 5 in itself by this electromagnetic field to causeheat generation. In the case where the solution 5 in itself is allowedto generate heat, as described above, it is not necessary that thecrucible 6 in itself be allowed to generate heat.

In the present example, the electromagnetic heating system is adopted asthe heating mechanism 10, although heating may be performed by usingother systems. As for the heating mechanism 10, other systems, forexample, a system in which the heat generated from a heating resistor,e.g., carbon, is transferred can be adopted. In the case where this heattransfer system is adopted, the heating resistor is arranged (betweenthe crucible 6 and the heat insulating material 8).

The seed crystal 4 is brought into contact with the solution 5 by atransfer mechanism 13. The transfer mechanism 13 has also a function ofcarrying out a crystal grown on the lower surface 4B of the seed crystal4. The transfer mechanism 13 is formed from the holding member 2, apower source 14, and the like. The seed crystal 4 and the crystal grownon the lower surface 4B of the seed crystal 4 are carried in and out bythis holding member 2. The seed crystal 4 is attached to a lower endsurface 2A of the holding member 2, and the movement of this holdingmember 2 in the vertical (D3, D4) direction is controlled by the powersource 14. In the present example, the D4 direction refers to thedownward direction in a physical space, and the D3 direction refers tothe upward direction in a physical space.

As shown in FIG. 2, the seed crystal 4 is fixed to the lower end surface2A of the holding member 2 with the adhesive 3 therebetween. It isenough that the holding member 2 has the lower end surface 2A. The shapeof the lower end surface 2A when viewed in plan is, for example, apolygonal shape, e.g., a tetragonal shape, or a circular shape. Thethree-dimensional shape of the holding member 2 is a bar, a rectangularparallelepiped, or the like.

The area of the lower end surface 2A may be either larger than the areaof the upper surface 4A of the seed crystal 4 or smaller than the areaof the upper surface 4A. In the present example, the area of the lowerend surface 2A is smaller than the area of the upper surface 4A of theseed crystal 4. In the case where the area of the lower end surface 2Aof the holding member 2 is larger than or equal to the area of the uppersurface 4A of the seed crystal 4, the whole upper surface 4A of the seedcrystal 4 can be fixed interposing the adhesive 3. Consequently, peelingof the seed crystal 4 from the holding member 2 can be furthersuppressed.

Meanwhile, the holding member 2 is made from carbon. It is enough thatthe holding member 2 is made from a material containing carbon as aprimary component. The holding member 2 is made from a polycrystal ofcarbon, a sintered body of carbon, or the like.

As for the holding member 2, start of rotation, acceleration ofrotation, deceleration of rotation, and stop of rotation are possible.The seed crystal 4 fixed to the lower end surface 2A of the holdingmember 2 is rotated or stopped by the holding member 2 being rotated orstopped. The seed crystal 4 is rotated in the D1 and D2 directions. Theseed crystal 4 is rotated in such a way that, for example, thebarycenter when viewed in plan is in the vicinity of the center ofrotation. The seed crystal 4 can be rotated clockwise orcounterclockwise when viewed in plan from the D3 direction. The seedcrystal 4 may be rotated clockwise or counterclockwise in such a waythat the number of revolutions becomes, for example, 500 rpm or less.

The coil 11 is formed of a conductor and is wound around thecircumference of the crucible 6. The alternating current power supply 12is to pass an alternating current through the coil 11, and the heatingtime of the inside of the crucible 6 to a predetermined temperature canbe reduced by employing an alternating current with a high frequency.

In the crystal production apparatus 1, the alternating current powersupply 12 of the heating mechanism 10 and the power source 14 of thetransfer mechanism 13 are connected to a control portion 15 and arecontrolled by control portion 15. That is, in the crystal productionapparatus 1, heating and temperature control of the solution 5 andcarrying in and out of the seed crystal 4 are ganged and controlled bythe control portion 15. The control portion 15 includes a centralprocessing unit and a storage device, e.g., memory, and is formed from aknown computer, for example.

<Method for Producing Crystal>

A method for producing a crystal, according to an embodiment of thepresent invention, will be described. The method for producing acrystal, according to the present embodiment, includes a step to bringthe lower surface 4B of the seed crystal 4 into contact with thesolution 5 (contact step), a step to rotate the seed crystal 4 (seedcrystal rotation step), a step to rotate the crucible 6 (cruciblerotation step), and a step to decelerate rotation of the crucible 6 and,thereafter, decelerate rotation of the seed crystal 4 while the lowersurface 4B of the seed crystal 4 is in contact with the solution 5(crucible stop step).

A crystal made of silicon carbide can be produced by the crystalproduction apparatus 1. The crystal production apparatus 1 includesmainly the crucible 6, the crucible container 7, the heating mechanism10, the transfer mechanism 13, and the control portion 15. In thecrystal production apparatus 1, a crystal is grown by using a solutiongrowth method. In this regard, FIG. 3 shows an outline of rotations ofthe crucible 6 and the seed crystal 4. The vertical axis indicates thenumber of revolutions (rpm) and the horizontal axis indicates theelapsed time. A broken line indicates changes in the number ofrevolutions of the crucible 6 with time, and a solid line indicateschanges in the number of revolutions of the seed crystal 4 with time.Also, T0 to T5 indicate elapsed times.

(Step to Bring Lower Surface of Seed Crystal into Contact with Solution)

The holding member 2 is moved downward and, thereby, the lower surface4B of the seed crystal 4 fixed to the lower end surface 2A of theholding member 2 is brought into contact with the solution 5. It isenough that at least the lower surface 4B of the seed crystal 4 is incontact with the solution 5. Specifically, the whole seed crystal 4 maybe immersed in the solution 5, part of the seed crystal 4 may beimmersed in the solution 5, or only the lower surface 4B of the seedcrystal 4 may be in contact with the solution 5.

The temperature of the solution 5 is lowered in the vicinity of thelower surface 4B of the seed crystal 4 by bringing the lower surface 4Bof the seed crystal 4 into contact with the solution 5, and a crystal ofsilicon carbide is precipitated on the lower surface 4B of the seedcrystal 4 where thermal equilibrium corresponds to a border line. Thatis, a crystal is grown on the lower surface 4B of the seed crystal 4taking advantage of precipitation of a crystal on the basis of thermalequilibrium.

After a crystal has begun to grow on the lower surface 4B of the seedcrystal 4, the holding member 2 is pulled upward gradually. The crystalof silicon carbide grown on the lower surface 4B of the seed crystal 4can be continuously grown in the thickness direction by pulling theholding member 2 upward. In that case as well, it is necessary that thelower surface 4B of the seed crystal 4 be in contact with at least thesolution 5. In this regard, in the case where the crystal is grown onthe lower surface 4B of the seed crystal 4, it is enough that the lowestend of the crystal is in contact with the solution 5. Here, the term“the lower surface 4B of the seed crystal 4” includes the lowest end ofthe crystal grown on the lower surface 4B of the seed crystal 4.

(Step to Rotate Seed Crystal)

Rotation of the seed crystal 4 is started. For example, the number ofrevolutions is increased in a predetermined time and the rotation of theseed crystal 4 is brought into a steady rotation state in which apredetermined number of revolutions is maintained. The rotationdirection of the seed crystal 4 can be set to become clockwise orcounterclockwise. In the present embodiment, the seed crystal 4 isrotated clockwise and, as shown in FIG. 3, the rotation is brought intoa steady rotation state at T1. In this regard, in FIG. 3, the positivedirection of the number of revolutions is specified to be clockwise andthe negative direction of the number of revolutions is specified to becounterclockwise.

(Step to Rotate Crucible)

Rotation of the crucible 6 is started. For example, the number ofrevolutions is increased in a predetermined time and the rotation of thecrucible 6 is brought into a steady rotation state in which apredetermined number of revolutions is maintained. In the presentembodiment, the crucible 6 is rotated clockwise and, as shown in FIG. 3,the rotation is brought into a steady rotation state at T1. In thisregard, in the present embodiment, the seed crystal 4 and the crucible 6are set to come into the steady rotation states at T1 at the same time.However, the steady rotation states may not be reached at the same time.

The number of revolutions of the crucible 6 in the steady rotation statecan be set to become smaller than, for example, the number ofrevolutions of the seed crystal 4 in the steady rotation state. In thesteady rotation states, for example, scattering of the solution 5outward from the crucible 6 because of a centrifugal force can besuppressed by allowing the number of revolutions of the crucible 6 tobecome smaller than the number of revolutions of the seed crystal 4.

The rotation direction of the crucible 6 can be set to become clockwiseor counterclockwise. The rotation direction of the crucible 6 can be setto become, for example, the same rotation direction as that of the seedcrystal 4. In the case where the crucible 6 and the seed crystal 4 arerotated in the same rotation direction, in the vicinity of the liquidsurface of the solution 5, the centrifugal force generated by therotation of the seed crystal 4 can be enhanced, and upward convectiongenerated in the solution 5 can be enhanced.

On the other hand, the rotation direction of the crucible 6 can be setto become the direction reverse to the rotation direction of the seedcrystal 4. In this case, the whole solution 5 is mixed easily and,therefore, variations in concentration distribution of carbon or siliconin the solution 5 can be suppressed. As a result, generation ofdislocation or micropipe of the crystal grown on the lower surface 4Bcan be suppressed and the quality of the crystal can be improved.

The above-described contact step, seed crystal rotation step, andcrucible rotation step may be performed in any order. That is, in thecase where the contact step is performed before the seed crystalrotation step and the crucible rotation step, the height positions ofthe seed crystal 4 and the crucible 6 can be adjusted in the state inwhich the seed crystal 4 and the crucible 6 are stopped. Consequently,the height position of the lower surface 4B of the seed crystal 4relative to the solution 5 can be controlled easily.

(Step to Decelerate Rotation of Crucible)

Rotation of the crucible 6 is decelerated and, thereafter, rotation ofthe seed crystal 4 is decelerated while the lower surface 4B of the seedcrystal 4 is in contact with the solution 5. That is, as shown in FIG.3, deceleration of the crucible 6 is started before rotation of the seedcrystal 4 is decelerated. The crucible 6 may be decelerated in such away that the number of revolutions is reduced gradually. The crucible 6is decelerated at a steady rate in such a way that a specific number ofrevolutions is reduced in a predetermined time.

When the crucible 6 is stopped, it is enough that the seed crystal 4 isrotated. For example, the crucible 6 is decelerated, and after therotation of the crucible 6 is stopped, rotation of the seed crystal 4 isstopped. The seed crystal 4 may be decelerated in such a way that thenumber of revolutions is gradually reduced in a predetermined time aswith deceleration of the crucible 6.

In the present embodiment, rotation of the crucible 6 is deceleratedwhile the seed crystal 4 is rotated. As shown in FIG. 4, an upwardconvection C1 which moves away from the bottom 6A of the crucible 6toward the lower surface 4B of the seed crystal 4 can be generatedeasily in the solution 5 in the crucible 6 by reducing the number ofrevolutions of the crucible 6. The reason for easy generation of theupward convection C1 will be described below.

When the number of revolutions of the crucible 6 is reduced, thesolution 5 located in the vicinity of the bottom 6A of the crucible 6 isdecelerated because of the friction against the bottom 6A. The flow rateof the convection C1″ in the vicinity of the bottom 6A becomes small ascompared with the flow rate of the convection C1′ in the upper portionof the crucible 6 because of the deceleration of the solution 5 locatedin the vicinity of the bottom 6A of the crucible 6. The upwardconvection C1 which flows upward from the vicinity of the bottom of thecrucible 6 can be generated easily by the above-described convection C1′and convection C1″.

On the other hand, the seed crystal 4 is rotated while being in contactwith the solution 5. As shown in FIG. 5, a centrifugal force is appliedto the vicinity of the surface of the solution 5 because the seedcrystal 4 is rotated while being in contact with the solution 5, and aconvection C2′ is generated easily along the same direction as therotation direction R1 of the seed crystal 4. As a result, the convectionC2 which moves upward from the vicinity of the bottom of the crucible 6is generated because of the rotation of the seed crystal 4.

The convection C2 generated because of the rotation of the seed crystal4, as described above, amplifies the convection C1 generated because ofdeceleration of the crucible 6. As a result, an upward convection C3which moves toward the lower surface 4B of the seed crystal 4, as shownin FIG. 6, can be generated.

In the case where the upward convection C3 which moves toward the lowersurface 4B of the seed crystal 4 is generated, carbon is supplied to thevicinity of the lower surface 4B of the seed crystal 4 easily.Consequently, a high-quality silicon carbide crystal can be grown on thelower surface 4B of the seed crystal 4. Also, carbon is supplied to thevicinity of the lower surface 4B of the seed crystal 4 easily and,thereby, the vicinity of the lower surface 4B comes into the state ofbeing rich in carbon and silicon, so that the crystal growth rate can beimproved.

In addition, as shown in FIG. 6, the upward convection C3 which movestoward the vicinity of the lower surface 4B of the seed crystal 4becomes a convection which moves toward the inside wall surface 6B ofthe crucible 6, where the direction is changed by the lower surface 4Bof the seed crystal 4. Consequently, although the temperature of thesolution 5 in the vicinity of the seed crystal 4 is lower than thetemperature of the solution 5 in the vicinity of the bottom 6A, ahigh-temperature solution 5 located in the vicinity of the bottom 6A ofthe crucible 6 can be carried to the vicinity of the surface by thisconvection. As a result, lowering of the surface temperature of thesolution 5 can be suppressed.

Also, even in the case where miscellaneous crystals or seeds ofmiscellaneous crystals are generated and floated on the surface of thesolution 5, these miscellaneous crystals can be moved away from the seedcrystal 4, and adhesion or growth of miscellaneous crystals on the lowersurface 4B, the side surface, or the upper surface 4A of the seedcrystal 4 can be suppressed. That is, as shown in FIG. 6, thetemperature of the solution 5 can be brought close to a uniformtemperature (soaking) by generating the convection C3 in the solution 5.

In the previously known method for growing a crystal, the temperature inthe vicinity of the surface is lowered because of radiation andvaporization of the solution and, thereby, the degree of supersaturationincreases, so that miscellaneous crystals or seeds of miscellaneouscrystals are generated in the vicinity of the surface of the solutioneasily and miscellaneous crystals adhere or grow around the seedcrystal. As a result, it is difficult to improve the quality of acrystal grown on the lower surface of the seed crystal.

(Modified Example 1 of Method for Producing Crystal)

In the deceleration step to decelerate the rotation of the crucible 6,the rotation of the crucible 6 may be stopped. That is, the rotation ofthe crucible 6 is decelerated and stopped while the seed crystal 4 isrotated. The rotation of the crucible 6 is decelerated by reducing thenumber of revolutions in a predetermined time and the rotation of thecrucible 6 is stopped completely.

Specifically, as shown in FIG. 3, the number of revolutions of thecrucible 6 is reduced over T2 to T3, and the rotation of the crucible 6is stopped at T3. It is enough that the seed crystal 4 is rotated whenthe crucible 6 is stopped (at T3 shown in FIG. 3). Therefore, the seedcrystal 4 may be rotated in the steady rotation state when the crucible6 is stopped or be on the way to reduction in the number of revolutions.In the present embodiment, the seed crystal 4 is rotated in the steadyrotation state when the crucible 6 is stopped.

The rotation of the crucible 6 is stopped while the lower surface 4B ofthe seed crystal 4 is in contact with the solution 5. It is enough thatthe lower surface 4B of the seed crystal 4 is in contact with thesolution 5 and, for example, the whole seed crystal 4 may be immersed inthe solution 5.

The rotation of the crucible 6 is stopped and, thereafter, the rotationof the seed crystal 4 is also stopped. Specifically, as shown in FIG. 3,the number of revolutions is reduced gradually over a predetermined time(from T4 to T5), and the seed crystal 4 is stopped completely at T5. Thestop time T5 of the seed crystal 4 is set to become after the stop timeT3 of the crucible 6.

(Modified Example 2 of Method for Producing Crystal)

A step of rotating the crucible 6 again (crucible second rotation step)may be performed after the crucible rotation stop step. The cruciblesecond rotation step may be performed again immediately after thecrucible rotation stop step, or be performed after a lapse of apredetermined time. In the example shown in FIG. 7, after the cruciblerotation stop step, rotation of the crucible 6 is started again after alapse of a predetermined time. The “predetermined time” may bedetermined depending on the state of a convection in the solution 5after the rotation of the crucible 6 is stopped. For example, the startmay wait until the convection stops after the rotation of the crucible 6is stopped. In this regard, FIG. 7 shows an outline of rotations of thecrucible 6 and the seed crystal 4 as with FIG. 3. A broken lineindicates changes in the number of revolutions of the crucible 6 withtime, and a solid line indicates changes in the number of revolutions ofthe seed crystal 4 with time.

In the crucible second rotation step, when rotation of the crucible 6 isstarted and the rotation of the crucible 6 is accelerated, the solution5 in the vicinity of the bottom 6A flows to the inside wall surface 6Bside of the crucible 6 because of the centrifugal force. Consequently,as shown in FIG. 8, a convection C4 along the inside wall surface 6B ofthe crucible 6 is generated easily.

As described above, the crucible second rotation step is performed afterthe crucible rotation stop step and, thereby, a crystal can be growneasily in the vicinity of the end portions (end portions in D5, D6directions) of the lower surface 4B of the seed crystal 4.

Specifically, for example, as shown in FIG. 9 (a), a crystal 4 a, inwhich the film thickness in the vicinity of the center (center in D5, D6directions) is large as compared with the film thicknesses of endportions (end portions in D5, D6 directions), is grown easily on thelower surface 4B of the seed crystal 4. Consequently, in the case wherethe crucible second rotation step is performed after the cruciblerotation stop step, the convection C4 as shown in FIG. 8 is generatedeasily and, therefore, the convection C4 rich in carbon hits against aninclined surface 4 aA of the crystal 4 a easily. As a result, a crystalgrows from the inclined surface 4 aA of the crystal 4 a easily, and asshown in FIG. 9 (b), a crystal 4 b, in which the film thickness in thevicinity of the end portion of the crystal 4 a is large, can groweasily.

The lower end surface of the crystal grown on the lower surface 4B ofthe seed crystal 4 can become flattened by combining a step in which acrystal grows easily in the vicinity of the center of the lower surface4B and a step in which a crystal grows easily in the vicinity of the endportion of the crystal 4 a, as described above. Consequently, anoccurrence of bunching, e.g., a height difference, of the crystal grownon the lower surface 4B can be suppressed and the crystal can be grownwhile an occurrence of polymorphic variation or dislocation issuppressed. As a result, a long lengths of high-quality crystal can begrown.

In the above description, the case where the crucible second rotationstep is performed after the crucible rotation stop step has beenexplained. However, in the case where the rotation of the crucible 6 isnot stopped in the deceleration step, the crucible 6 may be acceleratedin the rotation direction as the crucible second rotation step.

In the crucible second rotation step, rotation may be performed in thesame direction as the original rotation direction, or rotation may beperformed in the direction reverse to the original rotation direction(rotation direction of the crucible 6 in the crucible rotation step).This crucible second rotation step may be performed after thedeceleration step or the crucible stop step, while the convection C3 isgenerated in the solution 5 or after the convection C3 is stopped.

In the case where the crucible 6 is rotated in the direction reverse tothe original rotation direction, the convection C1″ in the vicinity ofthe bottom 6A of the crucible 6 in the solution 5 can be made slowerwhile the convection C3 is generated in the solution 5, so that theupward convection C1 can be made faster. As a result, the crystal growthrate and the like can be further improved.

Also, in the case where the crucible 6 is rotated in the directionreverse to the original rotation direction, carbon or silicon containedin the solution 5 in the crucible 6 can be mixed easily, so that theconcentration distribution of carbon or silicon in the solution 5 can bemade nearly steady and a crystal with reduced dislocation and the likecan be grown. Meanwhile, the seed crystal 4 may remain rotated or bestopped once during the crucible second rotation step and thereafter.

In the crucible second rotation step, for example, the seed crystal 4may be rotated in the direction reverse to the rotation direction of thecrucible 6 before the rotation of the crucible 6 is started again. Inthis case, when the rotation of the crucible 6 is started again, therotation direction of the convection C1″ in the vicinity of the bottom6A of the crucible 6 becomes reverse to the convection C1′ in thevicinity of the center of the solution 5. Consequently, the convectionC1″ becomes faster than the convection C1′ and, thereby, the convectionC1 is generated. On the other hand, the seed crystal 4 is rotated, sothat in the upper portion of the solution 5, the convection C2 whichmoves toward the outside (direction toward the inside wall surface 6B ofthe crucible 6) in the upper portion of the solution 5 is generated bythe centrifugal force because of the rotation of the seed crystal 4.

As described above, the seed crystal 4 has been rotated when therotation of the crucible 6 is started again and, thereby, the convectionC1 and the convection C2 are combined in the solution 5, so that theupward convection C3 which moves toward the lower surface 4B of the seedcrystal 4 can be generated.

Also, the crucible 6 is rotated in the direction reverse to the rotationdirection of the seed crystal 4 and, thereby, the solution 5 is mixedeasily, so that the concentration distributions of carbon and siliconcan be improved. As a result, generation of dislocation or micropipe ofthe crystal grown on the lower surface 4B of the seed crystal 4 can besuppressed.

On the other hand, the rotation of the seed crystal 4 may be stoppedafter the crucible second rotation step is started. For example, whenthe rotation of the crucible 6 is started again, in the case where thecrucible 6 is rotated in the direction reverse to the original rotationdirection, the rotation direction of the seed crystal 4 may be changedto the same direction as the rotation direction of the crucible 6. Inthat case, the seed crystal 4 is stopped temporarily.

The rotation of the seed crystal 4 is stopped temporarily, so that in aregion in which mixing is difficult when the seed crystal 4 is steadyrotated, the flow is changed because of stop of the rotation and thewhole solution 5 is mixed easily. Consequently, the concentrationdistribution of carbon or silicon in the solution 5 can be made steady.As a result, the concentration distributions of carbon and silicon of acrystal grown on the lower surface 4B of the seed crystal 4 can beimproved, and generation of dislocation or micropipe can be suppressed.

(Modified Example 3 of Method for Producing Crystal)

In the crucible second rotation step, the crucible 6 may be brought intothe steady rotation state from the stop state in a time shorter than thetime elapsed by the crucible 6 being brought into the stop state fromthe steady rotation state in the crucible stop step. That is, theconvection C4 shown in FIG. 8 is generated favorably in a time shorterthan the time in which the upward convection C3 is generated. An exampleof timing of rotations of the crucible 6 and seed crystal 4 is shown inFIG. 10. In this regard, the present modified example is the case wherethe seed crystal 4 is rotated steady. Also, FIG. 10 shows an outline ofrotations of the crucible 6 and the seed crystal 4 as with FIG. 3. Abroken line indicates changes in the number of revolutions of thecrucible 6 with time, and a solid line indicates changes in the numberof revolutions of the seed crystal 4 with time. Also, T0 to T7 indicateelapsed times.

Here, as for “the time elapsed by the crucible 6 being brought into thestop state from the steady rotation state in the deceleration step orthe crucible stop step”, for example, the time of from T2 to T3 shown inFIG. 10 may be employed. Also, as for “the time elapsed by the crucible6 being brought into the steady rotation state from the stop state”, forexample, the time of from T6 to T7 shown in FIG. 10 may be employed.

Meanwhile, even when the crucible 6 is stopped in the crucible stopstep, the convection C3 in the solution 5 is generated continuously.That is, the convection C3 in the solution 5 does not always stop at thesame time with the stop of the rotation of the crucible 6. Therefore, asfor “the time elapsed by the crucible 6 being brought into the stopstate from the steady rotation state in the crucible stop step”, forexample, the time of from T2 to T3′ may be employed in accordance withthe state of the convection of the solution 5. Also, as for “the timeelapsed by the crucible 6 being brought into the steady rotation statefrom the stop state”, for example, the time of from T6′ to T7 may beemployed in accordance with the state of the convection of the solution5.

In the case where the crucible second rotation step is performed, theconvection C4, as shown in FIG. 8, is generated easily. Therefore, asilicon carbide crystal which usually grows easily in the lateral(perpendicular) direction with respect to the thickness direction growsfurther easily because of the convection C4. Consequently, the crystalcan be flattened, as shown in FIG. 9 (b), by performing the cruciblesecond rotation step in a time shorter than the time elapsed by thecrucible 6 being brought into the stop state from the steady rotationstate in the crucible stop step.

(Modified Example 4 of Method for Producing Crystal)

As shown in FIG. 11, in the crucible stop step, the rotation of thecrucible 6 is brought into the stop state (T6), in which the rotation isstopped, from the steady rotation state (T2), in which a predeterminedrotation is performed, in a time shorter than the time (from T0 to T1)elapsed by the crucible 6 being brought into the steady rotation state,in which a predetermined rotation is performed, from the stop state(T0), in which the rotation is stopped, in the crucible rotation step.The rotation of the crucible 6 is stopped in a short time and, thereby,the flow in the vicinity of the bottom 6A of the solution 5 isdecelerated sharply relative to the convection C1′. As a result, theflow rate of the convection C1 in the solution 5 can be increased. Also,FIG. 11 shows an outline of rotations of the crucible 6 and the seedcrystal 4 as with FIG. 3. A broken line indicates changes in the numberof revolutions of the crucible 6 with time, and a solid line indicateschanges in the number of revolutions of the seed crystal 4 with time.Also, T0 to T9 indicate elapsed times.

Meanwhile, a difference in the flow rate of the convection C1″ in thesolution 5 can be adjusted by adjusting the length of the stop time(from T2 to T6) of the crucible 6. As a result, the flow rate of theupward convection C3 which moves toward the lower surface 4B of the seedcrystal 4 can be adjusted.

In addition, the flow rate of the convection C2 in the upper portion ofthe solution 5 generated on the basis of the centrifugal force becauseof the rotation of the seed crystal 4 can be adjusted by adjusting thenumber of revolutions of the seed crystal 4. According to this as well,the flow rate of the upward convection C3 which moves toward the lowersurface 4B of the seed crystal 4 can be adjusted.

Also, as shown in FIG. 11, the rotation of the seed crystal 4 may beaccelerated gradually in the stop time (from T2 to T6) of the crucible6. Specifically, the number of revolutions of the seed crystal 4 may beincreased (from T8 to T9) in the stop time (from T2 to T6) of thecrucible 6. In the case where the rotation of the seed crystal 4 isaccelerated in the stop time (from T2 to T6) of the crucible 6, asdescribed above, the flow rate of the convection C2 generated in theupper portion of the solution 5 can be increased and the flow rate ofthe upward convection C3 which moves toward the lower surface 4B of theseed crystal 4 can be increased.

(Modified Example 5 of Method for Producing Crystal)

As shown in FIG. 12, a flow guide member 16 may be fixed and arranged onthe bottom 6A in the crucible 6 before the step to bring the seedcrystal 4 into contact with the solution 5.

A material having a melting point higher than that of the solution 5 canbe used for the flow guide member 16. Specifically, the flow guidemember 16 can be formed from, for example, the same material as that forthe crucible 6. In the case where the flow guide member 16 is formedfrom the same material as that for the crucible 6, the flow guide member16 can be formed integrally in working of the crucible 6 and, therefore,good productivity can be exhibited. Meanwhile, a material having amelting point higher than that of the solution 5 is suitable for theflow guide member 16 and, for example, yttrium oxide, zirconium oxide,magnesium oxide, calcium oxide, or the like can be used.

The flow guide member 16 may be formed integrally with the crucible 6 orbe fixed and arranged on the bottom 6A of the crucible 6 afterward. Inthe case where the flow guide member 16 is arranged on the bottom 6A ofthe crucible 6 afterward, the location of the flow guide member 16 canbe changed easily and, therefore, the degree of flexibility of designcan be increased.

As for the method for fixing the flow guide member 16 to the bottom 6A,for example, a method in which fixing is performed by an adhesivecapable of maintaining adhesion even in the solution 6 can be employed.An adhesive containing a material having a melting point higher thanthat of the solution 5 can be used as the adhesive and, for example, acarbon adhesive, a ceramic adhesive containing a ceramic material, e.g.,alumina or zirconium, can be used. As described above, the flow guidemember 16 is fixed and arranged on the bottom 6A of the crucible 6 and,thereby, the flow guide member 16 is in the shape of a protrusion fromthe bottom 6A of the crucible 6.

As for the flow guide member 16, for example, a polygonal shape, e.g., atetragonal shape, or a three-dimensional shape, e.g., a pyramid, atruncated pyramid, a circular column, a circular cone, or a circulartruncated cone, can be employed. The bottom of the flow guide member 16having the above-described shape is fixed to the bottom 6A of thecrucible 6. The height of the flow guide member 16 is set in such a waythat the location of the upper end portion of the flow guide member 16is apart from the lower surface 4B of the seed crystal 4. The heightfrom the bottom to the upper end portion of the flow guide member 16 canbe set to become, for example, less than or equal to one-half the heightfrom the bottom 6A of the crucible 6 to the liquid surface of thesolution 5.

As shown in FIG. 13, the flow guide member 16 is arranged at a locationoverlapping the seed crystal 4 in the bottom 6A in such a way as to havethe shape of a protrusion from the bottom 6A. It is enough that at leastpart of the flow guide member 16 is arranged to overlap the seed crystal4. As shown in FIG. 12, the flow guide member 16 is arranged in such amanner and, therefore, an upward convection E1 which moves toward thelower surface 4B of the seed crystal 4 can be generated in the solution5 easily. This will be described below in detail with reference to FIG.14.

As shown in FIG. 14, a side-surface convection E2 along the inside wallsurface 6B of the crucible 6 becomes a bottom convection E3 along thebottom 6A, and when the resulting bottom convections E3 overlap oneanother in the vicinity of the rotation center of the bottom 6A, thebottom convection E3 moves along the outer circumference of the flowguide member 16 because the flow guide member 16 is arranged and movesupward (D3 direction) easily. Consequently, the bottom convection E3above the flow guide member 16 has a vector which moves upward (D3direction) larger than a vector which moves laterally (D5, D6directions), and when overlapping occurs, cancelling out in the lateraldirection can become difficult. As a result, the upward convection E1which moves toward the lower surface 4B of the seed crystal 4 can beincreased and the growth rate of a crystal grown on the lower surface 4Bof the seed crystal 4 can be improved.

Meanwhile, in the case where the flow guide member 16 is made fromcarbon as with the crucible 6, the bottom convection E3 flows along theflow guide member 16 and, thereby, carbon is supplied from the flowguide member 16 to the bottom convection E3, so that the upwardconvection E1 which moves toward the lower surface 4B of the seedcrystal 4 can be brought into a carbon-rich state. As a result, thecrystallinity of the crystal grown on the lower surface 4B of the seedcrystal 4 can be improved.

On the other hand, as shown in FIG. 15, the flow guide member 16 may bearranged in such a way as not to overlap the rotation center 17 servingas the center of rotation when the crucible 6 is rotated. The rotationcenter 17 is synonymous with the rotation axis when the crucible 6 isrotated. In the case where the flow guide member 16 is arranged in sucha manner, as shown in FIG. 15, an upward convection E1′ which movestoward the lower surface 4B of the seed crystal 4 can be furthergenerated from the vicinity of the rotation center 17 and, in addition,the solution 5 can be mixed. In this regard, a dotted line arrow shownin FIG. 15 schematically indicates the upward convection E1′ generatedwhen the crucible 6 is rotated.

As described above, the solution 5 can be mixed and, thereby, forexample, a stagnant place not mixed by the convection (E1, E2, or E3)generated in the solution 5 is mixed easily, so that variations inconcentration distribution of the whole solution 5 can be suppressed.Consequently, variations in composition of a crystal grown on the lowersurface 4B of the seed crystal 4 can be suppressed.

Meanwhile, as shown in FIG. 16, the cross-sectional area of the flowguide member 16 in the direction perpendicular to the thicknessdirection may decrease with increasing proximity to the end.Specifically, as shown in FIG. 16, the shape of a circular truncatedcone shown in (a), the shape of a semicircle shown in (b), or the shapeof a circular cone shown in (c) can be employed for the flow guidemember 16. In this regard, FIG. 16 shows sectional views cut in thedirection parallel to the thickness direction.

In the case where the flow guide member 16 has the shape of a circulartruncated cone (FIG. 16 (a)), the side surface is inclined at an acuteangle relative to the bottom, so that the bottom convection E3 can moveupward easily. Also, in the case where the flow guide member 16 has theshape of a semicircle (FIG. 16 (b)), bottom convections E3 can smoothlymove upward and overlap one another in the upper portion easily. Also,in the case where the flow guide member 16 has the shape of a circularcone (FIG. 16 (c)), the end is thin, so that the bottom convections E3move upward along the side surface and, thereafter, overlap one anotherin a state in which movement in the lateral direction is reduced.Therefore, cancelling out of the bottom convections E3 in the lateraldirection can be suppressed.

The shape of the flow guide member 16 is changed in such a manner and,thereby, cancelling out of the bottom convections E3 in the lateraldirection becomes further difficult, so that the upward convection E1which moves upward can be further increased. As a result, the growthrate of a crystal grown on the lower surface 4B of the seed crystal 4can be increased.

(Modified Example 6 of Method for Producing Crystal)

As shown in FIG. 17, the temperature of the solution 5 may be changedwith a lapse of time during growth of the crystal. Specifically, asshown in FIG. 17, when a crystal is grown by bringing the lower surface4B of the seed crystal 4 into contact with the solution 5, after a lapseof a predetermined time, the temperature of the solution 5 is raised toincrease the solubility of the solution 5. In this regard, in FIG. 17,the horizontal axis indicates the elapsed time, and the vertical axisindicates the temperature of the solution 5.

A raised temperature Te is favorably set at a temperature lower than theboiling point of silicon. Therefore, The raised temperature Te can beset at, for example, 2,100° C. or higher and 2,300° C. or lower. Thetiming to reach the raised temperature Te can be set in such a way thatthe interval of the raised temperature Te becomes, for example, 4 hoursor more and 10 hours or less.

In the case where a crystal is grown on the lower surface 4B of the seedcrystal 4, miscellaneous crystals may become large and granular in thesolution 5. However, the solubility of the solution 5 can be increasedby raising the temperature of the solution 5 to the raised temperatureTe on the way to growth, and granular miscellaneous crystals can bedissolved. Consequently, it is possible that granular miscellaneouscrystals do not adhere to the vicinity of the seed crystal 4 and, as aresult, the crystal can grow over a long period of time.

Also, in the example shown in FIG. 17, the temperature is raised and,thereafter, the temperature is lowered gradually with a lapse of time.When a crystal is grown, the carbon concentration in the solution 5decreases with a lapse of time. Consequently, the solubility of thesolution 5 can be reduced by lowering the temperature with a lapse oftime, and reduction in crystal growth rate can be suppressed.

On the other hand, as shown in FIG. 18, the temperature may be raisedgradually to the raised temperature Te, at which the solubility of thesolution 5 is high, with a lapse of time. It is possible that granularmiscellaneous crystals are not formed easily by allowing the solubilityto become high with a lapse of time, as described above. Consequently,the crystal can grow over a long period of time. In this regard, in FIG.18, the horizontal axis indicates the elapsed time, and the verticalaxis indicates the temperature of the solution 5.

The invention claimed is:
 1. A method for producing a crystal, where thelower surface of a seed crystal which is rotatably arranged and made ofsilicon carbide is brought into contact with a solution of siliconsolvent containing carbon in a crucible which is rotatably arranged andthe seed crystal is pulled up and a crystal of silicon carbide is grownfrom the solution on the lower surface of the seed crystal, comprising:bringing the lower surface of the seed crystal into contact with thesolution in a contact step; rotating the seed crystal in a seed crystalrotation step; rotating the crucible in a crucible rotation step; anddecelerating rotation of the crucible before decelerating rotation ofthe seed crystal, while the lower surface of the seed crystal is incontact with the solution in a deceleration step.
 2. The method forproducing a crystal according to claim 1, wherein a crucible rotationstop step stops rotation of the crucible in the deceleration step. 3.The method for producing a crystal according to claim 2, furthercomprising the step of rotating the crucible again in a crucible secondrotation step after the crucible rotation stop step.
 4. The method forproducing a crystal according to claim 3, wherein in the crucible secondrotation step, the crucible is rotated in a direction reverse to adirection in the crucible rotation step.
 5. The method for producing acrystal according to claim 3, wherein in the crucible second rotationstep, the crucible is brought into a steady rotation state from a stopstate in a time being shorter than a time elapsed to bring the crucibleinto the stop state from the steady rotation state in the cruciblerotation stop step.
 6. The method for producing a crystal according toclaim 1, wherein the crucible is rotated in one direction in thecrucible rotation step, and the seed crystal is rotated in the samedirection in the seed crystal rotation step.
 7. The method for producinga crystal according to claim 1, wherein the crucible rotation step andthe deceleration step are performed while the seed crystal is rotated.8. The method for producing a crystal according to claim 2, wherein inthe crucible rotation stop step, the crucible is brought into a stopstate from a steady rotation state in a time being shorter than a timeelapsed to bring the crucible into the steady rotation state from thestop state in the crucible rotation step.
 9. The method for producing acrystal according to claim 1, further comprising the step of fixing andarranging a flow guide member, which protrudes from an inside bottom ofthe crucible, on the inside bottom of the crucible at a positionoverlapping the seed crystal and being separated from the lower surfaceof the seed crystal in an arrangement step before the contact step. 10.The method for producing a crystal according to claim 9, wherein in thearrangement step, the flow guide member is arranged not overlapping therotation center of the crucible.
 11. The method for producing a crystalaccording to claim 9, wherein in the arrangement step, the flow guidemember, which has a cross-sectional area in the direction perpendicularto a thickness direction decreasing toward the top, is arranged.